Hydrogen cyanide

Hydrogen cyanide (HCN), also known as prussic acid, is a colorless, volatile, and extremely poisonous chemical compound whose vapors have a bitter almond odor. It melts at -14C and boils at 26C. It is produced in large quantities all over the world by the chemical industry where it is used in tempering steel, dyeing, explosives, engraving, the production of acrylic resin plastic, and other organic chemical products. A simple way to produce the gas is by reduction of potassium ferrocyanide solution.

The carbon atom (C) is triple bonded to the nitrogen atom (N), and in solution hydrogen cyanide converts to the cyanide ion CN^–.

A concentration of 300 parts per million of HCN will kill a human in a few minutes. It kills all aerobic organisms by shutting down the electron transport chain in the inner membrane of the mitochondrion: cyanide ion binds stronger to the metal in cytochrome a3 than oxygen does, preventing this cytochrome from combining electrons with oxygen. Furthermore, cyanide ion also binds to the iron atom in hemoglobin, preventing the dellivery of oxygen to the cells in the first place.

Zyklon B, the pesticide used in German gas chambers during the Holocaust, works by delivering hydrogen cyanide gas. Hydrogen cyanide is also the compound used in US execution gas chambers.

Cyanide was stockpiled in both the Soviet and the United States chemical weapons arsenals in the 1950s and 1960s. During the Cold War, the Soviet Union was thought to be planning to use hydrogen cyanide as a "blitzkrieg" weapon to clear a path through the opposing front line, knowing that the harmful gas itself would evaporate and allow unprotected access to the captured zone.